A family of integrable and non-integrable difference equations arising from cluster algebras

Abstract

The one-parameter family of second order nonlinear difference equations each of which is given by $$ x_{n-1}x_nx_{n+1}=x_{n-1}+(x_n)^{β-1}+x_{n+1} \qquad(β\in\mathbb{N}) $$ is explored. Since the equation above is arising from seed mutations of a rank 2 cluster algebra, its solution is periodic only when $β\leq3$. In order to evaluate the dynamics with $β\geq4$, algebraic entropy of the birational map equivalent to the difference equation is investigated; it vanishes when $β=4$ but is positive when $β\geq5$. This fact suggests that the difference equation with $β\leq4$ is integrable but that with $β\geq5$ is not. It is moreover shown that the difference equation with $β\geq4$ fails the singularity confinement test. This fact is consistent with linearizability of the equation with $β=4$ and reinforces non-integrability of the equation with $β\geq5$.